For only the second time since 9/11, America's defense secretary didn't visit U.S. troops in a war zone during December, breaking a long-standing tradition of personally and publicly thanking service members in combat who are separated from their families during the holiday season.

Pentagon boss Jim Mattis, who spent more than four decades in the Marine Corps and served in Iraq and Afghanistan, made a five-day trip through the Middle East in early December. He stopped in Kuwait and Pakistan — countries adjacent to Iraq and Afghanistan — but didn't cross the borders to see troops at war in either country. Last week, he visited troops in Guantanamo Bay, Cuba, and at military bases in Florida, North Carolina and South Carolina, wishing them holiday cheer.

It has been 15 years since a U.S. defense chief didn't travel to a war zone during the festive season. And the only time a holiday visit was skipped since Americans began fighting in Afghanistan was in December 2002. That year, then-Secretary Donald Rumsfeld went to a command post in Qatar that would be used a few months later to coordinate the launch of the Iraq war.

Asked recently why he wasn't going to Iraq or Afghanistan, Mattis said he didn't want to discuss his travel. "I carry out my duties to the best of my ability," said Mattis, who visited Iraq and Afghanistan earlier this year.

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Dana White, his chief spokeswoman, said the secretary "wanted the troops to enjoy their holiday uninterrupted. He is aware of the logistical challenges of a senior leader visit, especially in a war zone."

Defense secretary trips historically have been aimed at boosting troop morale, letting service members know that senior leaders and the U.S. public recognize their sacrifice.

And generals who have chaired the Joint Chiefs of Staff have routinely done their own December trips to war zones, taking celebrities on their flights as part of a USO entertainment tour.

It is less of a tradition for U.S. presidents to make December visits to conflict zones. Such trips require much greater logistical and security planning.

President George W. Bush visited Afghanistan twice and Iraq four times, including a secret Thanksgiving voyage to Baghdad in 2003 and a trip to both nations' capitals in December 2008. President Barack Obama flew to Iraq once as commander in chief and four times to Afghanistan. Only a December 2010 trip came during the holidays.

President Donald Trump hasn't yet gone to the war front, but Vice President Mike Pence flew to Afghanistan last week.

Less than three months after U.S. troops invaded Afghanistan, Rumsfeld flew into Bagram Air Base under high security, telling service members the World Trade Center "is still burning as we sit here." In two subsequent years, Rumsfeld went to Afghanistan and Iraq on Christmas Eve.

Rumsfeld's successor, Robert Gates, maintained the tradition, traveling to the war zone around the holidays during each of his five years in office.